Exclusive radio transmission and reception



Dec. 24, 1929. F. K. VREELAND 1,740,964

EXCLUSIVE RADIO TRANSMISSION AND RECEPTIQN Filed Sept. 15. 1922 g INVENTOR A TTORNE Y5 Patented ll 24, 1929 FREDERICK K. VREELAND, OF MONTCLAIR, NEW JERSEY EXCLUSIVE RADIO TRANSMISSION AND RECEPTION Application filed September 13, 1922. Serial No. 587,909.

The invention herein described resides in a method and system of transmission and re ception of radiant energy, particularly of radio telephonic signals, wherein the transmitted signals are unintelligible as received by an ordinary receiving apparatus but are clearly distinguished by a special receiver.

In carrying out the ,invention I employ waves of two different frequencies, emitted by the transmitting system, the two waves being modulated in opposite senses, so that their combined integrated effect is substantially constant. The two emitted waves have preferably nearly the same frequency, so that their combined effect on an ordinary receiver, which is incapable of discriminating between them, is either substantially zero or a beat effect resulting from the frequency difference. In receiving these waves I employ a pair, of

highly discriminating receiving systems each of which is receptive to one of the transmitted frequencies and practically eliminates the other. The oppositely modulated waves received by these two receiving systems are integrated by a detector of equivalent means, and applied to a receiving device in opposite senses, so that the effect of the reversed modulation is again reversed and the two integrated waves thereby produce a cumulative effect on the receiver, and the articulation is completely restored.

The highly discriminative receiver includes preferably an interference preventer of the type set forth in my United States Patents 1,500,476, July 8, 1924; 1,682,026, August 28, 1928; and 1,692,877, November 27, 1928. By means of the interference preventer each of the two receiving systems is made to exclude waves of one of the two frequencies while waves of the other frequency are received without interference. By this means I am able to secure a high degree of discrimination, so that transmitted frequencies may be em ployed which are so nearly alike that they cannot be distinguished by an ordinary tuned system. The transmission is thus inarticulate egicept when received by an especially designed receiver.

In the drawing Figure 1 is a schematic diagram of a transmitting and receiving appara tus embodying the invention.

Figure 2 shows an arrangement including an alternative means of producing the reversed modulation of the transmitted waves, and a simplified form of receiving apparatus.

Referring to Figure 1, T is the transmitting apparatus and R is the receiving apparatus. 1n the transmitting apparatus To and T1 are two generators of modulated radio frequency currents of different frequencies, which are associated with radiating means, such as an antenna A having a double oscillation period, which in the arrangement shown is produced by means of the differently tuned brances B and B The two radio generators or transmitters TT and Tr are controlled differentially by the same audio frequency modulating source. That is the modulation of the two transmitted waves is performed in opposite senses by the modulation control, which may be a differentially acting microphone or other equivalent device. In the arrangement specifically shown in Fig. 1 the modulation control means is a microphone M having two buttons, D, E, which are opposite- 1y affected by the vibration of the diaphragm. These opposite effects are impressed upon the transmitters T and T7 respectively by modulating transformers or other suitable means T T When the two transmitting generators are in operation the effect of the differential modulation described is to produce opposite variations in the transmitted waves of the two different frequencies, so that an increase in amplitude of one wave is accompanied by a decrease of amplitude of the other. If the two radiated waves are of the same initial amplitude and are equally modulated, as they are preferably, the combined integrated effect of the two waves is zero. IVh en the two transmitted frequencies are only slightly different their effects on an ordinary tuned receiver, which does not discriminate between them, will be equal and opposite, so that articulation cannot be observed. There will, however, be a beat effect due to the difference of frequency of the two transmitted waves. If the wave frequency is sufficiently high this beat note may be made above the audible limit, in which case signals are entirely inaudible, though it may properly be a note of audible frequency in which case the observed effect will be an inarticulate sound.

To receive the transmitted signals intelligibly I use a two frequency receiver, such as that shown schematically at R, Figure 1, which comprises two receiving units, each responsive to one of the frequencies and including means for excluding the other frequency. Thus in the arrangement shown A is a receiving antenna or collector having two degrees of freedom receptive to the two transmitted frequencies. This is accomplished in the arrangement shown by means of the two branch circuits B and B Associated with each of these branches is a highly selective tuned circuit F and F respectively, each with an associated detector or integrating device D D The output circuits of these detectors are connected differentially to a combining circuit N including a receiving or translating device 0.

Associated with each of the two receiving circuits is an interference preventer IP and IP respectively, whose function is to eliminate from each circuit effects of the frequency received by the other. These interference preventers may be of any one of the several designs described in my aforementioned ap plications, or of other suitable type, the ob- ]ect being to secure efiicient separation of the two transmitted frequencies. This is done in the arrangement referred to by deriving from the interfering current a balancing current of interfering frequency, determining its phase so that its effect on the receiving circuit'is opposite to the interfering effect, and balancing these effects in intensity and phase to mutual annulment. With the type of interference preventer described an eflicient sep aration is effective even for very small frequency differences, since the phasing adjustment causes very perfect annulment of the interfering frequency without diminishing signals of the other frequency.

The two transmitted waves being completely separated by their respective receiving circuits F F with associated interference preven-ters IP and 11, they are integrated by the detectors D D and applied in opposite senses to the combining circuit N, where the effect of the differential modulation in the transmitter is rectified and a cumulative effect is produced, reproducing the original articulation.

The arrangement shown in Fig. 2 includes in the transmitting apparatus a modified arrangement for producing the differential modulation, and includes in the receiving apparatus a simplified form of interference preventer.

Here M, as in Fig. 1,is the modulation control means, which is here. shown as a telephone In the receiving apparatus is ,a specialized form of interference preventer. As is fully explained in the above mentioned applications, my interference preventer 1n 1ts varlous forms includes the following elements: a balancing circuit tuned to the interfering frequency, so associated with the collector circuit that currents of interfering frequency are set up therein, means for phasingthe balancing current so that its effect is in phase opposition with the effect of the interfering current, and means for balancing the effects of these two currents in intensity and phase to mutualannulment. In the arrangement of Fig. 2 two interference balancing circuits are combined with the two receivlng circuits in such -a manner that several functions of the receiving and balancing systems are combined in certain elements, thereby greatly simplifying the apparatus. In the particular arrangement shown A is a receiving antenna, which is shown with a single tunin means B and B as it is usually suflicient 0 have a single tuned antenna when the two frequencies are sufliciently close together. F and F are the two receiving circuits tuned respectively to the two transmitted frequencies. I and I are interference balancing circuits, also tuned to the two frequencies respectively. Each of the balancing circuits, I and I is coupled to both of the secondary receiving circuits F and F Detectors of any deslred form or associated with the two receiving circuits F and F and the they act cumulatively thereon. The opera I tion of the apparatus is as follows:

The collecting circuit A being tuned to be receptive to both of the transmitted frequencies, the receiving circuits F and F are tuned respectively to thetwo frequencies. The balancing circuits I and I are then tuned approximately to these frequencies respectively. Signals of the frequency 1 will be received normally by the secondary circuit F and its associated apparatus. -This circuit will also be more or less affected by currents of the frequency 2, by virtue of the imperfect selectivity of the collector and secondary circuits. The interfering impulses thus impressed on the circuit F are annulled by balancing impulses derived through the secondary circuit F and the balancing circuit I Both of these circuits being tuned to the frequency 2, the signal current of frequency 2 in the secondary circuit F will set up a balancing current of the same frequency in the balancing circuit- I Because of the double quadration in passing from the collector circuit A to the receiving circuit F and then to the balancing circuit I the balancing current in I will be approximately in opposition to the interfering current of frequency 2 in the collector circuit. Precise opposition is secured by a slight phasing adjustment of the condenser C in the balancing circuit, and preciseequality of intensity is obtained by adjusting one of the couplings, for example the coupling M Similarly the effect of interfering impulses of frequency 1 oaths secondary receiving circuit F is balanced by a balancing current of frequency I in the balancing circuit I derived from the signal current in the secondary circuit F and a precise balance is secured by a phasing adjustment by means of the condenser C and intensity adjustment through the coupling M By the use of this arrangen'ient complete separation of the two received frequencies is secured with greatly simplified apparatus and adjustments.

lVhile I have described at length certain circuit arrangements whereby the invention may be carried out I do not wish to limit myself to any particular arrangement of circuits since the invent-ion can be carried out successfully in a variety of ways. Neither is the invention limited to any particular frequencies in the transmitted waves. I prefer, however to employ frequencies so close together that they cannot be discriminated by ordinary tuning means, yet sufficiently separated to avoid the production of an audible beat note. Under these conditions the transmitted signals will not produce any observed effect on an ordinary tunedreceiver. For example, if one of the transmitted waves has a wave length of 150 meters and a corresponding frequency of 2,000,000 cycles per second, and the second frequency differs from this by for example the frequency being 2,010,000 cycles, the beat frequently will be 10,000 cycles per second, which is so near the audible limit as to be practicably inaudible. Lower frequencies and larger or smaller frequency differences may be employed, in which case a beat note may be observed in an ordinary receiver, but by the highly discriminating receiving means here described the articulation will be clearly audible without a beat note.

It will be understood that the same apparatus may be used for transmitting telc graphic code signals as well as for telephony, by substituting a key or other control means for the modulating microphone and its associated apparatus.

What I claim is- 1. In a transmitting system for radiant energy, two simultaneously operating generators of different radio frequencies, radiat-i ing means associated with said generators, modulating means respectively associated with each generator and radiator, an audio frequency modulation control source and means operating in opposite senses for controlling the two modulating means from this source and providing simultaneous and opposite modulations in the radiations produced by the two generators.

2. In a transmitting system for radiant energy, two simultaneously operating generators of different radio frequencies, radiating means associated with said generators,

modulating means respectively "associated with each generator and radiator, a telephone transmitter, and means for operating the two modulating means in opposite sense by the telephone transmitter and providing simultaneous and opposite modulations in the ra diations produced by the two generators.

3. In a receiving system for radio intelligence, two receivers responsive to different frequencies, means for excluding from each receiver impulses of the other or interfering frequency, said means including a balancing circuit tuned to interfering frequency and means for balancing the effect of currents in this circuit against the interfering effect in intensity and phase, means for separately receiving the unbalanced signal effects of the two frequencies, and means for applying these effects to a receiver in opposite senses.

4. The method of transmitting and receiving radio intelligence which consists in generating and radiating two wave trains of different frequencies, modulating the two transmitted wave trains similarly and in opposite senses, receiving the two transmitted waves separately, balancing the interfering effect of each received wave on the other by an equal and opposite effect derived. from the same source, and combining the separately received waves in opposite senses to produce a cumulative effect.

5. The method of receiving two radio waves of different frequencies which consists in receiving selectively a current of each frequency, deriving from each received current a balancing current of the same frequency, balancing mutual interference of the two currents by equal and opposite effects derived from the two balancing currents, and receiving the unbalanced signal effects.

6. In a receiving system for radio waves of two different frequencies, collecting means, two tuned receiving circuits responsive respectively to the two wave frequencies, interference balancing means for neutralizing the interfering effect of each frequency on the receiving circuit of the other frequency, and means for receiving the unbalanced signal effects.

This specification signed this 18th day of August A. D. 1922.

FREDERICK K. VREELAND. 

